Machines are known for excavating underground tunnels which include at least one digging head mounted at the front of a structure provided with means for intermittent advance, so as to progress at the pace of the excavation of the tunnel. Generally this structure is tubular and surrounds an internal conveyor allowing the spoil to be taken away to the rear.
In ground which is soft or only moderately hard, it is necessary to place a lining for the walls of the tunnel behind the excavation machine so as to protect the people working there against falls. In practice such a lining is made up of discontinuous elements such as arches, centerings, or plates put in place and assembled as soon as there is room. Carrying out this lining is therefore done in discontinuous stages and since it is difficult to do the lining work while the machine is operating, the latter must be stopped frequently so as not to leave too great a length of the tunnel unlined. Moreover it is of its nature that the positioning of the lining is carried out when the men working at it are unprotected.